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Benefit Plans and Other Postretirement Benefits (Tables)
 3 Months Ended
Mar. 31, 2018
Retirement Benefits [Abstract]  
Components of Net Periodic Benefit Cost (Credit)
Components of Net Periodic Benefit Cost (Credit)
 
 
Three Months Ended March 31
 
 
2018
 
2017
 
2018
 
2017
(Thousands of Dollars)
 
Pension Benefits
 
Postretirement Health
Care Benefits
Service cost
 
$
2,430

 
$
2,440

 
$
279

 
$
219

Interest cost (a)
 
4,603

 
4,928

 
410

 
415

Expected return on plan assets (a)
 
(7,082
)
 
(6,971
)
 
(615
)
 
(589
)
Amortization of prior service credit (a)
 
(35
)
 
—

 
(101
)
 
(100
)
Amortization of net loss (gain) (a)
 
3,517

 
3,245

 
(113
)
 
(155
)
Net periodic benefit cost (credit)
 
3,433

 
3,642

 
(140
)
 
(210
)
Credits not recognized due to the effects of regulation
 
974

 
148

 
—

 
—

Net benefit cost (credit) recognized for financial reporting
 
$
4,407

 
$
3,790

 
$
(140
)
 
$
(210
)